Britannica Web sites

A calendar is a tool used to mark the passing of time. People of ancient times based their calendars on the most obvious regular events they knew-the changing positions of the sun, moon, and stars. These calendars helped them figure out when to plant and harvest their crops. Over time different groups of people developed other calendars based on their own needs and beliefs.

A calendar is a system for dividing time over extended periods, such as days, months, or years. People have kept track of the days by the march of daylight and darkness and of the changing seasons in order to know when to plant crops and to get ready for winter. Sometimes they kept the record by notching a stick or knotting a cord once every day. They also watched the changing positions of the Sun and stars, the changes of the Moon, and the habits of plants and animals. The making of an exact calendar, however, has perplexed humankind for ages because the natural divisions of time by days (Earth cycle), months (lunar cycle), and years (solar cycle) do not fit together perfectly.